This invention relates to a switched power supply operating with current mode regulation.
A switched power supply operating with current mode regulation may include a switching transistor which is coupled to the primary winding of a transformer and to a DC input voltage derived from the AC mains supply. The switching transistor is periodically switched between conduction and cutoff by a control voltage. The control voltage is supplied, for example, from an oscillator connected on the primary side of the transformer, or is generated on the secondary side of the transformer and then transferred to the primary side.
During the on-time of the switching transistor, magnetic energy is stored in the transformer from the input voltage source in the form of an upramping current flowing in the primary winding of the transformer. After the switching transistor is cut off, this stored magnetic energy is transferred via an induced voltage pulse to the secondary windings of the transformer. A corresponding charging current passes into the filter capacitors of the secondary side rectifier circuits.
The energy stored in the transformer is discharged into the secondary side rectifier circuits during the off-time of the switching transistor until current in the transformer secondary windings decreases to zero. Thereafter, a deadtime interval occurs when both the switching transistor and the rectifier circuits are nonconductive. The deadtime is terminated when the switching transistor is again switched into the conductive state and the transformer is again charged with magnetic energy in the form of current flowing in the primary winding.
During normal operation, the transformer is in the discharged state when the switching transistor is turned on. During malfunction or abnormal operation, a situation may arise where the discharge current of the transformer flows for a relatively long period in the secondary supply rectifier circuits and does not decrease to zero before the end of the off-time of the switching transistor. Such a situation may arise when the secondary side rectifier circuits are excessively loaded, when a short circuit is developed, or when components in the regulator control circuit fail. In this malfunction situation, the switching transistor undesirably becomes conductive to recharge the transformer before the transformer has been completely discharged. Such an operating mode is undesirable and may be hazardous because of the excessive currents and voltages that may be generated by the transformer.
Switched power supplies with current mode regulation typically employ one of two regulating schemes. One scheme requires that the switching transistor operate with a substantially constant on-time, and that the switching frequency be varied in a negative feedback fashion to control the output voltage. In the other scheme, the switching frequency is maintained substantially constant, and the on-time of the switching transistor is varied in a negative feedback fashion to control the output voltage. Regulation by varying frequency can present certain problems if the frequency crosses certain boundaries, for example, corresponding to the horizontal scanning frequencies f.sub.H and 2f.sub.H, where f.sub.H is approximately 15,734 Hz for an NTSC interlaced video signal. The greater the range of regulation necessary, the more likely the variable frequency is to cross one or more of the problem causing boundaries. The range of regulation is a function of the variables which must be compensated, for example, output power in the range of 10 watts to 100 watts and supply voltage in the range of 180 volts to 250 volts. Existing switched mode power supplies with variable frequency regulators cannot operate over such a range without crossing the boundaries. Regulation by varying frequency can also present problems under very light loading conditions, for example during standby operation. The operating frequency under these conditions can be undesirably high. On the other hand, a variable frequency regulator can respond quickly to overload and short circuit conditions by rapidly decreasing switching frequency, without necessarily turning off power to the load completely.
Regulation by constant frequency offers the advantage of operation at a frequency likely to undergo only small variations at most, and in any event too small to cross any of the problem causing boundaries. Regulation by constant frequency also offers the advantage of preventing high frequency operation under light loading conditions. However, regulation by constant frequency can be difficult to implement in conjunction with an overload protection scheme, which must override the tendency of the regulator to supply more and more energy to the load as the output voltage falls to levels indicative of overload or short circuit conditions. Even if the sense of the pulse width modulation were reversed during overload, so that the pulse width or on-time was reduced, the power would be reduced until no power was produced at all, although a complete shutdown due to a temporary overload condition may not be desirable. Very little range would be available between normal operation and complete shutdown. Moreover, regulation by constant frequency can be inappropriate during power supply start-up.